Research on visual short-term memory debates whether a fixed number of “slots” or instead the precision of maintained information restricts performance. We tested participants on visual-orientation working memory. For each trial a brief sample display presented two or four oriented bars within each visual hemifield; one hemifield was precued for retention at trial start. After a delay of 1.5 seconds, participants judged whether a single probe bar was rotated clockwise or anticlockwise from the sample orientation shown at that location prior to the delay. The change in orientation could require more precision (15 degrees) or less precision (45 degrees) for the comparison. The color of the sample display indicated on each trial which precision was most likely. Behaviorally we found that rarer trials with 30 degrees of actual change (occurring on one third of trials) were judged better when more rather than less precision was anticipated; but only with the lower set-size of 2 items. This shows that the precision retained can be varied at will, provided set-size is low. Two related EEG experiments revealed that the well-known “contralateral delay activity” (CDA) component during the delay period was higher not only for set-size four than two, but also higher for two items in the high-precision than low-precision conditions. These results support the emerging notion of dynamic allocation of limited capacity in visual working-memory, while showing that CDA can provide a neural marker for retained precision as well as retained set-size.